WordPress at a glance
action is not described

signup_extra_fields action-hook . WP 3.0.0

Fires at the end of the user registration form on the site sign-up form.

Usage

add_action( 'signup_extra_fields', 'action_function_name_5806' );
function action_function_name_5806( $errors ){
	// action...
}
$errors(WP_Error)
A WP_Error object containing 'user_name' or 'user_email' errors.

Where the hook is called

show_user_form()
signup_extra_fields
wp-signup.php 254
do_action( 'signup_extra_fields', $errors );

Where the hook is used (in WP core)

Does not used.